In fibre-optic systems the optical signal is attenuated, inter alia in fibres and branches, and the systems are therefore equipped with amplifiers at suitable spacing to compensate for the attenuation. The amplification can take place in optical semiconductor amplifiers, e.g. of the kind described in IEEE SPECTRUM, MAY 1984, Soichi Kobayashi and Tatsuya Kimura: "Semiconductor optical amplifiers". These amplifiers essentially include a semiconductor diode, both end surfaces of which are semi-reflecting, so-called Fabry-Perot amplifiers or antireflection treated, so-called travelling wave amplifiers and are driven by an electric current or a light source. The travelling wave amplifiers have the disadvantage of generating wideband noise during the amplification, but have the advantage of accepting large tolerances in the wavelengths of an incoming signal. A bandwidth filter must therefore by installed so that an acceptable signal-to-noise ratio is obtained. A Fabry-Perot amplifier, with reflecting end surfaces may serve as such a filter and by selecting the reflectivity of the end surfaces in a suitable way a compromise between noise filtration and wavelength tolerance is obtained. This filter has the disadvantage that the transmitted signal is amplified during the filtration and is reflected back again along the incoming optical fibre, thus causing the possibility of instability in the transmission system. This instability can also occur in amplifiers with anti-reflection treated end surfaces since it is difficult to achieve a perfect anti-reflection treatment. If the instability is to be prevented, the reflected signal must be attenuated, which requires so-called optical insulators to be installed between the amplifiers. This makes the transmission systems complicated since the insulators cannot be built integrated with the amplifiers.